The ability of some viral and tumor DNAs to induce foci of NIH/3T3 cells has been widely used for the identification and characterization of onc genes. It has become apparent, however, that the focus forming assay is capable of detecting only a small subset of the potentially active tumorinducing genes, and furthermore, that focus formation is a relatively undefined phenotype yielding little information about the biochemical basis of transformation. To expand the range of detectable oncogenes and to provide information about the biochemical basis of transformation, an alternative selective system has been developed. The assay involves selection for the ability of cells to grow in serum-free medium lacking either insulin or platelet-derived growth factor, both of which are required to support the growth of NIH/353 cells. Preliminary data indicate that growth factor independence is a property distinct from loss of contact inhibition and a limited set of onc genes tested to date show unique patterns in their ability to relieve NIH/3T3 cells of their growth-factor requirements. Transfer of human tumor DNA into NIH/3T3 cells demonstrated that DNAs which did not induce focus formation did confer growth factor independence. The experiments in the proposal utilize the system for the characterization of known onc genes, or the detection and isolation of additional onc genes from human tumors and tumor cell lines, and for the isolation of genes which partially revert the transformed phenotype of tumor cell lines. (X)